Load Dyno vs Inertia Dyno


Load Dyno vs Inertia Dyno

There are two main types of chassis dynamometers for measuring your truck’s power – inertia dynos and load dynos.

Inertia Dyno
As the name indicates, an inertia dyno measures horsepower with inertia, or the velocity of a fixed mass (the rollers).  So, the faster a truck can accelerate the rollers, the more power it has. Then, torque is displayed as a calculation from the horsepower figures (Torque = Horsepower X 5252 / RPM).  Since inertia dynos are a somewhat simple design they are often less expensive, and thusly more common.

The second type of dyno is a load bearing dyno. 

Load Dyno
As you can guess a load dyno uses load to measure power.  The two main ways to apply load or resistance to the dyno rollers is with an eddy current brake, or a water brake.  The amount of force that the truck applies to the rollers is measured by a strain gauge in Ft-Lbs, or torque.  Horsepower is then calculated from the torque reading (HP = Torque X 5252 / RPM).   Due to additional hardware required for a load style dynos, they are generally more expensive to purchase and install than inertia dynos.

There are dynos located throughout the world at various altitudes and climate conditions.  Even the same dyno can experience vastly different environmental conditions from one season to the next.  This is where correction factors come in.  A correction factor can be applied to the raw dyno data to compensate for elevation and atmospheric conditions.  This way you can compare numbers made on a dyno at high elevation on a hot humid summer day to those from a sea level run made on a nice 60 degree day.  The most commonly used is the SAE correction factor, which can be applied to both inertia and load dynos.

Which type of dyno is better, inertia or load?  While it is an arguable topic, many would agree that a load dyno is better for several reasons.  First, applying a load similar to what the truck sees on the street will show a more accurate power curve.  Also, applying additional load can be useful for simulating towing situations, as well as for diagnostics.

An inertia dyno often “feels” lighter and doesn’t simulate as accurately how the truck reacts on the street or track.  Some trucks may not achieve full boost on an inertia dyno which can be a problem if it is tuned to only deliver full fuel at maximum boost.  That being said, inertia dynos are still a viable option for consistently measuring maximum horsepower and torque for less of an investment than a load dyno.

Check out this post for some additional information on dynos:


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